1. Field of the Invention
The present invention relates to a ladder-type filter, and in particular, to a ladder type filter for use in mobile communication devices such as automobile telephones, portable telephones and other suitable devices, a branching filter, and a communication device.
2. Description of the Related Art
In recent years, reductions in size and weight of communication devices such as portable telephones have been required, and composite communication devices have been developed. Dielectric filters having superior electrical characteristics and withstand power properties have been used in branching filters and top filters according to known techniques. Recently, surface acoustic wave (SAW) filters having reduced sizes and weights have been more required for the branching filters and the top filters. Especially, for filters which are used on the transmission sides of branching filters, high withstand power properties have been required. Moreover, dielectric filters having low insertion losses in pass-bands and high attenuations outside the pass-band (blocking-band) are in great demand.
Referring to the surface acoustic wave filters used on the transmission sides of branching filters, ladder type filters including one terminal-pair type surface acoustic wave resonators arranged in parallel arms and in series arms thereof disclosed in Patent Document 1 (Japanese Patent No. 2800905 and corresponding Japanese Unexamined Patent Application Publication No. 5-183380) are generally used. FIG. 45 shows the basic configuration of a ladder type filter including one terminal-pair type surface acoustic wave resonators 51 and 52. FIG. 3 shows the configuration of a surface acoustic wave resonator which defines the ladder type filter. FIG. 46 shows the typical characteristics of the ladder type filter.
Ladder type filters have a low loss and a wide band, and provide a large attenuation relatively near the pass-band thereof without difficulty. Thus, sufficient characteristics can be obtained. Ladder type filters have been widely used in portable telephones and suitable devices.
Electrode films having high withstand power properties which define SAW filters have been developed recently, and have been used as filters provided on the transmission side of branching filters. Branching filters, including ladder type SAW filters, which are operated by an AMPS system or by an 800 MHz band CDMA system have been used.
On the other hand, ladder type filters for use in portable telephones, especially for use in branching filters are required to have improved performance. Thus, such devices must have an even lower insertion loss and an even higher attenuation (also see Patent Document 2 which is Japanese Unexamined Patent Application Publication No. 9-167937, Patent Document 3 which is Japanese Patent No. 3191473 and corresponding Japanese Unexamined Patent Application Publication No. 6-260876, and Patent Document 4 which is Japanese Unexamined Patent Application Publication No. 2002-223147).
For example, in a PDC system using an 800 MHz band, filters provided on the transmission sides of the branching filters must have an insertion loss of up to about 1 dB and an attenuation of at least about 40 dB. This system has a transmission band in the range of 940 MHz to 958 MHz and a reception band in the range of 810 MHz to 828 MHz, and thus, the frequency interval between the transmission band and the reception band is 112 MHz. In the case of the above-described AMPS system and 800 MHz band CDMA system, the frequency interval between the transmission band and the reception band is 20 MHz. Accordingly, the frequency interval of the 800 MHz band PDC system is at least five times of that of the above-described AMPS system.
However, the attenuation frequency-range of a ladder type filter is determined by the frequency intervals between the resonant frequencies and the anti-resonant frequencies of resonators (one terminal-pair surface acoustic wave resonators) arranged in the series arm and the parallel arm. Attenuation can be achieved in a frequency band relatively near to the pass band, while attenuation cannot be achieved in a frequency band far from the pass-band.
Moreover, the attenuation is determined by the ratio of the capacitance of the resonator (one terminal-pair type surface acoustic wave resonator) arranged in the parallel arm to that of the resonator arranged in the series arm. Thus, the insertion loss and the attenuation have an inverse relationship to one another.
Therefore, problematically, the requirements that the pass-band and the blocking band are separated from each other by at least 100 MHz, the insertion loss in the pass-band is up to 1 dB and the attenuation in the blocking band is at least 40 dB cannot be satisfied with known ladder type filters.
The above-described problems also occur when ladder type piezoelectric thin-film filters are used, each including an Si substrate having an aperture or a concavity and a vibrating portion in which a pair of opposed upper and lower electrodes sandwich at least one layer of piezoelectric thin film (e.g., made of ZnO or AlN) from the upper and lower surfaces thereof, the piezoelectric thin film being arranged so as to cover the aperture or the concavity.